For instance, a hemodialysis apparatus includes a dialysis circuit which supplies and discharges a dialysate to and from a dialyzer. This dialysis circuit is provided with a quantitative vessel which houses a dialysate of a fixed quantity to be supplied to the dialyzer, and houses a waste dialysate of a fixed quantity that was discharged from the dialyzer. With a dialysis circuit, in a state where blood is caused to flow to a primary side of a dialysis membrane of the dialyzer, by discharging the dialysate from a secondary side of the dialyzer to the quantitative vessel while supplying the dialysate from the quantitative vessel to the secondary side of the dialysis membrane of the dialyzer, the dialysate is passed through the dialyzer, waste products contained in the blood are introduced into the dialysate, and the patient's blood is thereby subject to dialytic treatment (refer to Patent Document 1).
Moreover, in order to remove the excess water content contained in the patient's body, ultrafiltration treatment is performed in the foregoing dialytic treatment. In order to perform this ultrafiltration treatment, normally, an ultrafiltration line for removing the water content from the blood is connected to the waste fluid line that is more downstream than the dialyzer of the dialysis circuit. The ultrafiltration line is provided with, for instance, a pump, and the water content is sucked and removed from the blood inside the dialyzer by operating the pump (refer to Patent Document 1). Moreover, as another means for removing the water content from the blood, proposed is a method of providing a constant-pressure valve to a waste fluid line, maintaining a branch point of the waste fluid line and an ultrafiltration line to a positive pressure by the constant-pressure valve, and removing the water content of the blood in the dialysate through the ultrafiltration line (refer to Patent Document 2).    [Patent Document 1] Patent Publication JP-A-H5-146506    [Patent Document 2] Patent Publication JP-A-2000-126284
Meanwhile, in order to strictly management the amount of fluid contained in the patient's body, for instance, it is necessary to strictly control the fluid removal that is discharged from the ultrafiltration line. Thus, a quantitative pump is used as the pump of the ultrafiltration pump of foregoing Patent Document 1, and strictly controlled. Consequently, the control and the like of the hemodialysis apparatus become complicated and, for example, this causes the hemodialysis apparatus to become expensive. Moreover, with the constant-pressure valve of the dialysis circuit of foregoing Patent Document 2, not only does the control of the apparatus become difficult due to the control of the constant-pressure valve and the like, but also there is a possibility that the positive pressure at the branch point cannot be sufficiently maintained since the branch point of the waste fluid line and the ultrafiltration line is downstream of the dialyzer, and there may be cases where the fluid removal cannot be controlled with high precision.